A known electrophotographic color-image forming apparatus adopts a so-called in-line system having independent image forming units of different colors to achieve high-speed printing. This in-line color-image forming apparatus is configured to transfer images from the color image forming units to an intermediate transfer belt in sequence and to further transfer the images together from the intermediate transfer belt to a recording medium.
In such a color-image forming apparatus, color misregistration (positional misalignment) occurs due to a mechanical factor of the color image forming units when images are overlapped. In particular, with a configuration in which a laser scanner (optical scanning unit) and a photosensitive drum are provided for each of the color image forming units, the positional relationship between the laser scanner and the photosensitive drum differs from color to color, which hinders synchronizing laser scanning positions on the photosensitive drums, thus causing color misregistration. To correct such color misregistration, the above color-image forming apparatus performs color-misregistration correction control. PTL 1 discloses an image forming apparatus that performs color-misregistration correction control by transferring color toner images for detection from photosensitive drums onto an image bearing member (an intermediate transfer belt or the like) and by detecting the relative position of the detecting toner images in a scanning direction and a conveying direction using an optical sensor.
However, the detection of detecting toner images with the optical sensor in the known color-misregistration correction control in the related art has the following problem. That is, since detecting toner images (a density of 100%) are transferred from the photosensitive drums to the image bearing member (belt) for color-misregistration correction control, it takes much time and effort to remove them, thus reducing the usability of the image forming apparatus.